Steel

Steel is iron with a little carbon, usually well under 1% for sheet and structural steel, plus small amounts of other elements. That small dose of carbon makes iron much stronger. The world makes close to two billion tonnes a year.

Most new steel starts as pig iron from a blast furnace, which holds about 4% carbon and is brittle. The molten pig iron is poured into a basic oxygen furnace, usually with some scrap steel. A water-cooled lance blows pure oxygen onto it at supersonic speed, burning off most of the carbon as gas and turning other impurities into slag. A batch of a few hundred tonnes takes well under an hour. Then alloying elements are added. Manganese is the most common: it adds strength and toughness and ties up leftover sulfur that would make the steel crack. The steel is then cast and rolled.

In this build, steel becomes galvanized steel, screws, the frame of the speaker, the core of the stand, the can of an alkaline battery, connector shells and the springs in the remote control. Roughly 30% of the world's steel is made instead by melting scrap in electric arc furnaces.